Fluid swivels are commonly used in offshore installations to transfer gas and oil between a fixed underwater pipeline and a tanker that may drift around the installation. A typical fluid swivel includes ring-shaped inner and outer walls forming an annular chamber between them and forming a pair of gap passages extending from opposite sides of the chamber to the environment. A primary and second seal are placed along each gap passage, the secondary seal serving as backup in the event of failure of the primary seal. An isolation seal with substantially zero pressure difference across it, is often placed between the chamber and each primary seal, with a clean fluid lying between the isolation seal and the primary seal. This allows the primary seal, which must withstand a large pressure difference, to be bathed in clean fluid.
Some newer fluid swivel designs use axial, or face seals for the primary and secondary seals. In order that the axial forces on the inner and outer wall remain the same before and after failure of a primary seal, the primary and secondary seals are positioned the same distance from the axis of the swivel and both have high pressure ends that face in the same radial direction (either both towards the axis or both away from the axis). Such an arrangement results in a convoluted gap passage portion extending between the primary and secondary seals. The fluid swivel must be assembled by stacking the parts on one another in a predetermined sequence to account for the fact that there is radial interfernce between parts of the inner and outer wall. The convoluted gap passage results in the need for a considerable number of different parts in the fluid swivel. An increased number of parts increases the expense of the fluid swivel because of the need to form additional parts and because tighter tolerances are required where a greater number of parts are used. Fewer parts also decrease the potential leak paths in the swivel. A fluid swivel which minimized the number of separate parts would be of considerable value.
The isolation seal, which lies between the chamber and the primary seal, often experiences a shortened lifetime because of the buildup of sediment in the form of sand, pipescale, rust, etc. on the side facing the chamber. A design which minimized such buildup would also be of considerable value.